1. Field of the Invention
The present invention relates to a method for producing a silica-alumina fiber. In particular, the present invention relates to a method for producing silica-alumina fibers comprising steps mixing polyaluminoxane with a specific amount of a silicon-containing compound and a specific aliphatic carboxylic acid, spinning the resulting spinning liquid to obtain a precursor fiber and calcining the precursor fiber to obtain a silica-alumina fiber having good mechanical strength and heat resistance, which method is excellent in spinnability and spinning.
2. Description of the Related Art
As methods for producing a silica-alumina fiber, there are known an inorganic salt method, a slurry method, a sol-gel method and a precursor polymer method.
The inorganic salt method comprises the steps of adding a water-soluble polymer such as polyethylene oxide of polyvinyl alcohol to an aqueous solution of an aluminum salt, mixing water-soluble polysiloxane or silica sol in the solution, and condensing and heat aging the solution to obtain a viscous spinning solution. Since this method imparts the viscosity to the solution by the water-soluble organic polymer, the solution is gelled in the preparation step of the solution, so that the spinning solution lacks stability.
The slurry method add a crystalline growth suppressing agent, a binder component and an organic polymer such as polyethylene oxide to fine alumina particles and uses the resulting viscous slurry as a spinning liquid. In this method, since a particle size of the solid particles, their amounts and a state of dispersion have influences on the spinning property and the mechanical properties of the spun fiber, their control is very difficult.
The sol-gel method condenses an alumina sol containing an ion such as HCOO.sup.- or CH.sub.3 COO.sup.- and a silica sol to form a viscous spinning liquid. In this method, since the components tend to precipitate, the solution becomes turbid or the viscosity suddenly increases in the condensation step, the control of condensation is difficult.
The precursor polymer method prepares a spinning liquid by mixing a silicon-containing compound such as a silicate ester with polyaluminoxane. In this method, since homogeneity of the spinning liquid is high and neither the organic polymer nor the binder is necessary, the obtained fiber is characterized in its high mechanical strength. But, since the polyaluminoxane is easily hydrolyzed, the spinning stability is low, and the strength of the obtained fiber varies greatly. Then, a modified precursor polymer method is proposed. For example, in the method disclosed in Japanese Patent Publication No. 13768/1976, polyaluminoxane and a silicon-containing compound are mixed in the presence of a solvent to obtain a spinning liquid containing 50 to 60 % by weight of the polyaluminoxane, spinning the spinning liquid and calcining the spun fiber. In particular, this Patent describes that the spinnability is improved by the addition of an organic polymer such as polyethylene glycol, polyvinyl formal and the like or a higher aliphatic carboxylic acid such as stearic acid, palmitic acid and the like to the spinning liquid.
Japanese Patent Publication No. 49362/1987 discloses that polymetalloxane having a phenoxy group improves the spinnability of the spinning liquid greatly.
However, since the spinning liquids used in the methods of U.S. Pat. No. 4,101,615 and Japanese Patent Publication No. 49362/1987 contain a large amount of a solvent, they have some problems that control of the viscosity of spinning liquid is difficult so that the spinning liquid may not be stably spun, and that the precursor fibers stick to each other during the spinning so that the mechanical properties of the obtained fibers fluctuate. In addition, volatilized solvents should be collected during the ,spinning, or it is necessary to provide a drying step for removing the solvent from the precursor fibers. Thereby, the production cost is increased.